A seasonal precipitation isoscape for New Zealand

Region: New Zealand Focus: Application of a seasonal precipitation isoscape for New Zealand, to incorporate influences of regional scale atmospheric circulation, and local topography. New hydrological insights: Our regionally calibrated sinusoidal model demonstrated substantial variation in the strength (amplitude and fit) of seasonal cycles of precipitation isotope values across New Zealand. Amplitudes of sine models fitted to monthly precipitation delta 18O records from 51 sites ranged from 0.38 to 3.14 parts per thousand and were lowest in coastal and windward areas. Offsets (central tendencies) of seasonal cycles predictably decreased at higher elevations and latitudes, but the best interpolation models appear to capture some of the orographic effects present across New Zealand's South Island. Phase (lag) of seasonal cycles was generally lower in the south and east of the country. We suggest that seasonal cycles of precipitation stable isotope values in some areas of New Zealand (but not others) are conducive to calculation of catchment transit times and may require consideration for interpreting sources of recharge to groundwater and river water. Uncorrected delta 18O isoscapes produced by interpolation of sine parameters offered better fits to monthly precipitation delta 18O values than previous model predictions available for New Zealand, and subsequent residual correction further improved isoscape performance. These results have wider implications for the application of stable isotopes as environmental tracers in regions with mixed marine- and continental-type climates.